1. Field of the invention
This invention relates generally to devices for the heating of water for general service water applications and for use in a hydronic space heating system. In particular, the invention relates to a module for rapidly heating water for service hot water and space heating with high thermal efficiency.
2. Description of the Prior Art
Space heating systems for residences and other building all operate on the same general principle. The air in the space to be heated circulates in a closed loop system. Part of the loop includes a heat exchanger, either one in direct contact with a burning fuel or one in contact with a heat transfer fluid, such as water, which has been heated by a burning fuel.
One type of space heating system, a direct forced air system, uses a furnace containing a combustion chamber in which fuel is burned creating combustion gases which then pass through the interior of a heat exchanger. Air from the space to be heated passes around the exterior of the heat exchanger absorbing heat by means of conduction and convection before being returned to the heated space.
Another type of heating system, a hydronic system, uses a furnace containing a combustion chamber and a heat exchanger for transferring the heat of combustion to a heat transfer liquid, usually water, which is then circulated through a closed loop system to a second heat exchanger where the heat in the loop is transferred to the air in the space to be heated. This second heat exchanger may be either a radiator, located in the space, over which the air to be heated passes by natural convective flow or a heating coil located in an air box having a fan to circulate air from the space to be heated over the coil and back to the space.
Conventional heating systems of both the direct forced air and hydronic types are relatively inefficient, as the combustion gases exit the furnace heat exchanger and are exhausted to the exterior of the building via a flue or chimney while still at a relatively high temperature. Much of the heat energy contained in the input fuel is therefore lost "up the flue" and is not available to heat spaces within the building. Conventional systems suffer from "up the flue" losses even when they are not producing heat by combustion. Warmed air from the heated space rises and is exhausted to the atmosphere via the flue and stack. In a conventional hydronic system, the off cycle loss is exacerbated because the hot water in the tank surrounding the flue heats the air in the flue, increasing the rate of space air loss and reducing tank water temperature.
In a conventional hydronic system, the water heating volume in the furnace is relatively small, resulting in a relatively small heat storage capability. The burner and heat exchanger in such a system must therefore be relatively large in order to meet the nearly instantaneous demand when temperature control devices in the spaces to be heated call for heat. This in turn requires a relatively large furnace enclosure to house the burners in such a system.
In conventional hot water heaters, a burner of relatively small capacity is used to heat a relatively large volume of water. The burner in such a heater is not able to heat the water rapidly enough to meet an instantaneous demand for hot water. Instead, such demands are met by drawing upon the relatively large reservoir of hot water maintained in the heater. High hot water demands may, however, deplete the reservoir. The efficiency of conventional hot water heaters is relatively low not only because of "up the flue" losses described in the discussion of space heating systems above, but also the ambient heat losses from the large reservoir of hot water maintained in such systems.
In recent years, the industry has developed and introduced improved high efficiency condensing furnaces. In such furnaces, "up the flue" heat losses are reduced by using the hot combustion exhaust gases to preheat the circulating ambient air before it enters the main heat exchanger. The preheating process reduces the temperature of the exhaust gases, some to their dewpoint and lower, and those gases condense. Some such furnaces have automatic flue dampers or other flow restricting devices that prevent or reduce the flow of warmed space air "up the flue" when the furnace is not operating.
Most residential and commercial building require both space heating and hot water. Such buildings now generally satisfy these requirements with separate space heating furnaces and hot water heaters. Not only does such an arrangement require a relatively large amount of space, it requires the provision of separate gas or fuel lines, separate ignition and control devices and, in many cases, separate or more complex exhaust gas flue systems to serve the separate heating devices, resulting in increased material and installation costs.
When in operation, burners used in conventional heating systems typically produce low frequency noise, known as "combustion roar". If the furnace is connected to a system of ducting, the ducts can amplify and transmit the sound generated in the furnace to spaces being heated.
The combustion gases generated by the burners normally used in conventional heating and hot water systems contain oxides of nitrogen, collectively termed NO.sub.x, which, together with other combustion gases, go "up the flue" to be vented to the atmosphere. Limiting the concentration of NO.sub.x in the flue gases is desirable, as heating devices sold in certain jurisdictions must comply with very low NO.sub.x emission requirements.
The radiant infrared type of burner overcomes the drawbacks of conventional burners as it burns silently and with substantially reduced concentrations of NO.sub.x in its combustion gases.
The industry has also developed heat exchangers of improved and more compact design, allowing them to be enclosed in smaller furnace enclosures.